In centrifugal evaporators samples are usually held in glass or plastic tubes or, sometimes, in a large number of small wells in plastic blocks. The sample holders are spun so as to arrange for a considerable centrifugal force to be applied to them in a direction which forces the liquid to the lower part of the sample tubes to prevent any frothing or spitting of the liquid out of the sample tubes when a vacuum is applied. The spinning samples are held in a vacuum-tight chamber (referred to henceforth as “chamber”) which is connected to a vacuum pumping device.
Evaporators of this type are well known and many types are available commercially. They all suffer from the difficulty of providing the latent heat of evaporation to the samples to allow evaporation at reasonably high speed without allowing the samples to reach temperatures which could damage or destroy the samples which are often thermolabile.
No heat can be provided by conduction because the samples are held in a vacuum but microwaves or radiant heat from a high temperature source (500°-3000° C.) can be used to provide the heat required for evaporation Radiation from a low temperature source, eg the chamber walls at 40° C., is frequently used but it cannot provide sufficient heat for rapid evaporation of anything other than small samples of highly volatile solvents. The use of radiant heat and of microwaves is known. Microwaves can cause damage to some samples and in the known embodiments that use radiant heat, the heat has been applied in a manner in which samples are not heated uniformly so that some samples can be dry whilst others are still liquid. This causes overheating of the dry samples if the heat is maintained long enough to dry the last samples.